* Add animation support for Doodad actors (Play Mode) into the core
engine, so that the Doodad script can register named animations and
play them without managing all the details themselves.
* Doodad API functions on Self: AddAnimation, PlayAnimation,
StopAnimation, IsAnimating
* CLI: the `doodad convert` command will name each layer after the
filename used as the input image.
* CLI: fix the `doodad convert` command creating duplicate Palette
colors when converting a series of input images into a Doodad.
